I would check your .dt, .Xauthority, etc... under your home directory if CDE kicks you back out. Also check the permissions on the location where /dev/null is linked, I have seen instances where /devices/pseudo/mm@0:null has permissions set to 440, it should be 666.
Are you running any kind of wrapper programs for Telnet and FTP. Also next time it stops responding, instead of rebooting I would try snooping the ports, running truss -p on the process id number, and checking /var/adm/messages for any clues...